Method and control device for adjusting the damping force of a shock absorber

ABSTRACT

A method for adjusting the damping force of shock absorbers, connected between a vehicle body and a wheel at vehicle corners of a motor vehicle, wherein at least one damping force which is referred to a center of gravity of the vehicle body and which is divided between the shock absorbers of the respective wheels of the motor vehicle is determined as a function of at least one variable which represents the movement of the vehicle body and/or a movement of the respective wheel. For at least one of the lifting, pitching and rolling modal directions, the respective damping force is determined as a function of a predefined, constantly maintained degree of damping of respective modal direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application No. 102016 123 420.6, filed Dec. 5, 2016, which is incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The invention relates to a method for adjusting the damping force of atleast one shock absorber, connected between a vehicle body and a wheelof a motor vehicle.

Furthermore, the invention relates to a control device for executing themethod.

BACKGROUND OF THE INVENTION

Methods for adjusting the damping force of shock absorbers of a motorvehicle are sufficiently known from practice. For example, according tothe Skyhook principle the adjustment of the damping force for a shockabsorber, connected between a vehicle body and a wheel, of a motorvehicle takes place in such a way that a damping force for therespective shock absorber is determined as a function of a movement ofthe vehicle body and/or as a function of a movement of the respectivewheel and is set, specifically within a defined actuation range.

In this context, according to practice, damping forces or damping torqueat the center of gravity of the motor vehicle are calculated for thelifting, pitching and rolling modal directions of movement. Dampingtorques can be converted into damping forces. The damping forces whichare calculated for the lifting, pitching and rolling modal directions ofmovement and are referred to the center of gravity of the motor vehicleare distributed between the axles of the motor vehicle and between thevehicle corners and therefore between the individual wheels and summed.The distribution of the damping forces which are referred to the centerof gravity of the motor vehicle between the axles and the vehiclecorners is carried out here on the basis of permanently predefinedparameters.

In methods for adjusting the damping force which are known frompractice, the damping forces for the lifting, pitching and rolling modaldirections of movement are adapted as a function of speeds and/oraccelerations. Until now, a changing mass of the motor vehicle andchanging spring stiffnesses have not been taken into account. This givesrise to an inconsistent oscillation behavior of the motor vehicle.

U.S. Pat. No. 4,916,632 A, which is incorporated by reference herein,and U.S. Pat. No. 5,944,763 A, which is also incorporated by referenceherein, have disclosed methods for adjusting the damping force of shockabsorbers which are known from the prior art.

SUMMARY OF THE INVENTION

Described herein is a method for adjusting the damping force for shockabsorbers of a motor vehicle, and a control device for executing themethod, which method and control device can be used to improve thequality of damping.

According to aspects of the invention, for at least one of the lifting,pitching and rolling modal directions, the respective damping force isdetermined as a function of a predefined, constantly maintained degreeof damping of the respective modal direction.

It is proposed for the first time to keep constant a degree of dampingwhich is subject per se to a change during operation owing to a changingvehicle mass and/or a changing spring stiffness. As a result, a constantdamping behavior or oscillation behavior of the motor vehicle can beensured, and the quality of damping can be improved.

For all the lifting, pitching and rolling modal directions, the dampingforce is preferably determined as a function of a predefined, constantlymaintained degree of damping of the corresponding modal direction. Thisis particularly preferred for making available a consistent oscillationbehavior of the motor vehicle and for improving the quality of damping.

According to an advantageous development, for the lifting modaldirection the degree of damping for the lifting modal direction is keptconstant, in that a damping constant for the lifting modal direction isadapted as a function of a changing lifting-spring stiffness and/orchanging mass of the motor vehicle, with the result that the degree ofdamping for the lifting modal direction remains constant, and/or for thepitching modal direction the degree of damping is kept constant in sucha way that a damping constant for the pitching modal direction isadapted as a function of a changing pitching-spring stiffness and/or achanging pitching moment of mass inertia of the motor vehicle, with theresult that the degree of damping for the pitching modal directionremains constant, and/or for the rolling modal direction the degree ofdamping is kept constant in such a way that a damping constant for therolling modal directions is adapted as a function of a changingrolling-spring stiffness and/or a changing rolling-moment of massinertia of the motor vehicle, with the result that the degree of dampingfor the rolling modal direction remains constant. This permits thedegrees of damping for the lifting, pitching and rolling modaldirections to be kept constant, specifically independently of changingspring stiffnesses and/or changing moments of mass inertia and/or achanging vehicle mass. This is particularly preferred in order to makeavailable a consistent oscillation behavior of the motor vehicle and toimprove the quality of damping.

The respective damping constant is calculated in a continuously updatedfashion in order to keep the degree of damping constant.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred developments of the invention can be found in the followingdescription. Exemplary embodiments of the invention are explained inmore detail with reference to the drawing without being restrictedthereto. In the drawing:

FIG. 1 shows a detail of a motor vehicle;

FIG. 2 shows a first time diagram clarifying the prior art;

FIG. 3 shows a second time diagram clarifying the prior art;

FIG. 4 shows a first time diagram clarifying the invention; and

FIG. 5 shows a second time diagram clarifying the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows, as a quarter vehicle model of a motor vehicle, a highlyschematic detail of a motor vehicle in the region of a wheel 10 of themotor vehicle and of a vehicle body 11 thereof, wherein according toFIG. 1 both the shock absorber 12 and a spring element 13 are connectedbetween the vehicle body 11 and the wheel 10.

According to FIG. 1, the damping force of the shock absorber 12 which isconnected between the wheel 10 and the vehicle body 11 can be adjusted.

In order to adjust the damping force, which is made available by theshock absorber 12, a damping force is determined, in particular by acontrol device of the motor vehicle, specifically as a function of atleast one variable which represents a movement of the vehicle body 11and/or as a function of at least one variable which represents amovement of the respective wheel 10.

Then, if the damping force is adjusted according to the so-calledSkyhook principle, a corresponding controller of the control devicedetermines a damping force as a function of at least one variable whichrepresents the movement of the vehicle body 11 and/or as a function ofat least one variable which represents a movement of the respectivewheel, specifically for a plurality of modal directions of movement ofthe vehicle body 11, specifically for modal lifting of the vehicle body11, modal pitching of the vehicle body 11 and modal rolling of thevehicle body 11. This damping force is firstly referred to the center ofgravity of the vehicle body 11 or of the motor vehicle and has to bedivided between the axles and edges.

Using a controller to carry out the basic determination, in particularaccording to the Skyhook method, of a setpoint damping force which isreferred to the center of gravity of the vehicle body 11 or motorvehicle is basically known to the person skilled in the art in questionhere.

At this point, for the sake of completeness reference will be made tothe fact that in Skyhook methods which are known from the prior art forthe lifting, pitching and rolling modal directions of movement,corresponding damping forces F_(LIFT), F_(PITCH) and F_(ROLL) as well astranslator modal speeds v_(LIFT), v_(PITCH) and v_(ROLL) are calculatedtaking into account the following equations:

F _(LIFT)=2D _(LIFT)√{square root over (c _(LIFT) m)}z,v _(LIFT) =z,

M _(PITCH)=2D _(PITCH)√{square root over (c _(PITCH) J _(PITCH))}φ,F_(PITCH) =f(M _(PITCH)),v _(PITCH) =f(φ),

M _(ROLL)=2D _(ROLL)√{square root over (c _(ROLL) J _(ROLL))}Θ,F _(ROLL)=f(M _(ROLL)),v _(ROLL) =f(Θ).

According to the prior art, these damping forces F_(LIFT), F_(PITCH) andF_(ROLL) at the center of gravity of the motor vehicle are calculatedand divided between the individual vehicle corners of the motor vehicleand therefore the individual wheels of the motor vehicle, and for eachvehicle corner are summed to form a total damping force for therespective vehicle corner.

For the above calculation of the damping forces F_(LIFT), F_(PITCH) andF_(ROLL) for the lifting, pitching and rolling modal directions, therespective degrees of damping D_(LIFT), D_(PITCH) and D_(ROLL) are takeninto account for the modal directions, for which degrees of dampingD_(LIFT), D_(PITCH), and D_(ROLL) the following relationships apply:

${D_{LIFT} = \frac{d_{LIFT}}{2\sqrt{c_{LIFT}m}}},{D_{PITCH} = \frac{d_{PITCH}}{2\sqrt{c_{PITCH}J_{PITCH}}}},{D_{ROLL} = {\frac{d_{ROLL}}{2\sqrt{c_{ROLL}J_{ROLL}}}.}}$

In this context, according to the prior art constant damping constantsd_(LIFT), d_(PITCH) and d_(ROLL) are predefined on the control side.

Changing spring stiffnesses c_(LIFT), c_(PITCH), c_(ROLL), a changingmass m and changing moments of mass inertia J_(PITCH) and J_(ROLL) haveup to now not been taken into account according to the prior art.

The effects of this procedure known from the prior art on theoscillation behavior of a motor vehicle in the case of a changing massand in the case of a changing spring stiffness for the lifting modaldirection are shown in FIGS. 2 and 3, wherein in FIGS. 2 and 3 theoscillation path z is plotted over the time t for the lifting modaldirection. An oscillation profile 14 respectively clarifies in FIGS. 2and 3 the oscillation behavior for the lifting modal direction which isformed when the actual spring stiffness for the lifting modal directionand the actual mass of the motor vehicle correspond to the correspondingvariables which are predefined on the control side, with the result thata decay behavior which is displayed by the curve profiles 15 is thenformed.

However, if the mass of the motor vehicle is increased, for exampleowing to an increase in its cargo, the curve profile 16 in FIG. 2 showshow an increased mass causes the oscillation profile or the oscillationbehavior to change, assuming a constant spring stiffness and constantdamping constant. In FIG. 2, it is apparent from the second harmonicthat the damping in the form of the chronological decay behavior and thedecay behavior per oscillation cycle becomes smaller.

The curve profile 17 in FIG. 3 illustrates a change in the oscillationprofile for the case in which the spring stiffness for the lifting modaldirection is increased, specifically assuming a constant mass of themotor vehicle and a constant damping constant. The spring stiffness canchange, for example, owing to suspension kinematics. Therefore, forexample in the case of air springs the spring stiffness can be changedby connecting and disconnecting air volumes.

The curve profile 17 in FIG. 3 shows how a change in spring stiffnessacts on the oscillation profile. Although a spring stiffness which isincreased in FIG. 3 does not have any influence on the decay over time,it does on the relative decay per oscillation cycle. Accordingly, itfollows from FIGS. 2 and 3 that the oscillation behavior of a motorvehicle for the lifting modal direction is dependent on a changingvehicle mass and a changing spring stiffness for the lifting modaldirection. A changing mass and a changing spring stiffness also bringabout a change in the damping requirement.

In the sense of the present invention here, for at least one of thelifting, pitching and rolling modal directions, preferably for all thelifting, pitching and rolling modal directions, the respective dampingforce F_(LIFT), F_(PITCH), F_(ROLL) are determined as a function of apredefined and constantly maintained degree of damping D_(LIFT),D_(PITCH), D_(ROLL). In this context, in the case of a changing vehiclemass and/or in the case of a changing mass inertia and/or in the case ofa changing spring stiffness for the respective modal direction, therespective damping constant is adapted with the result that the degreeof damping for the respective modal direction remains constant.

The damping constant for the lifting modal direction is preferablyadapted in such a way that the degree of damping for the lifting modaldirection remains constant, specifically

${D_{LIFT} = \frac{d_{LIFT}}{2\sqrt{c_{LIFT}m}}},$

whereD_(LIFT) is the degree of damping for the lifting modal direction,d_(LIFT) is the damping constant for the lifting modal direction,c_(LIFT) is the lifting-spring stiffness, andm is the mass.

The damping constant for the pitching modal direction is preferablyadapted in such a way that the degree of damping remains constant,specifically

${D_{PITCH} = \frac{d_{PITCH}}{2\sqrt{c_{PITCH}J_{PITCH}}}},$

whereD_(PITCH) is the degree of damping for the pitching modal direction,d_(PITCH) is the damping constant for the pitching modal direction,c_(PITCH) is the pitching-spring stiffness, andJ_(PITCH) is the pitching moment of mass inertia.

The damping constant for the rolling modal direction is preferablyadapted in such a way that the degree of damping for the rolling modaldirection remains constant, specifically

${D_{ROLL} = \frac{d_{ROLL}}{2\sqrt{c_{ROLL}J_{ROLL}}}},$

whereinD_(ROLL) is the degree of damping for the rolling modal direction,d_(ROLL) is the damping constant for the rolling modal direction,c_(ROLL) is the rolling-spring stiffness, andJ_(ROLL) is the rolling moment of mass inertia.

The invention relates to the underlying concept of adapting the dampingconstant d for the respective modal direction, specifically as afunction of a changing vehicle mass and/or of changing moments of massinertia and/or of changing spring stiffnesses, in order thereby to keepthe degree of damping constant. In this way, a consistent oscillationbehavior of the motor vehicle can be made available.

A changing spring stiffness can be determined, for example, as afunction of a characteristic curve. A changing mass can be either sensedby measuring the equipment using a load sensor or alternativelycalculated. The moment of mass inertia can be correspondingly scaled asa function of a changing vehicle mass, using the so-called Steiner'stheorum, with which the person skilled in the art in question isfamiliar.

FIGS. 4 and 5 provide evidence of the effectiveness of the methodaccording to aspects of the invention for the lifting modal direction,wherein in turn a plurality of time curve profiles are shown in FIGS. 4and 5 plotted against the time t, wherein the curve profiles 14 and 15in FIGS. 4 and 5 correspond to the curve profiles 14 and 15 in FIGS. 2and 3, that is to say show an oscillation profile 14 and a decaybehavior 15 for the case in which an actual mass and an actual springstiffness for the lifting modal direction correspond to variables whichare predefined on the control side.

In FIG. 4, the time curve profile 18 illustrates an oscillation profilewhich is formed when, in accordance with FIG. 2, the vehicle mass isincreased, for example owing to an increase in cargo, but according toaspects of the invention the damping constant d_(LIFT) is not keptconstant but instead is adapted in such a way that despite the changingvehicle mass the degree of damping D_(LIFT) remains constant.

As a result of this measure, the decay which is referred to theoscillation cycle can be kept unchanged compared to the variant of thecurve profile 14. The natural frequency which has become smaller withthe mass becomes slightly smaller as result of the increased dampingconstant.

FIG. 5 shows the effectiveness of the method in the case of a changingspring stiffness for the lifting modal direction, wherein the curveprofile 19 shows an oscillation profile for the case in which, byanalogy with FIG. 3, the spring stiffness of the lifting modal directionbecomes larger, but at the same time the damping constant d_(LIFT) isadapted in order to keep the degree of damping D_(LIFT) constant.

From FIG. 5 it is apparent that in the case of an oscillation profile 19which is formed by using the invention the decay which is referred tothe oscillation cycle remains unchanged compared to the profile 14. Thenatural frequency which has become larger as a result of the relativelyhigh spring stiffness is compensated slightly by increasing the springstiffness.

With the invention it is possible to ensure a consistent oscillationbehavior for changing vehicle masses and/or changing moments of massinertia and/or changing spring stiffnesses. As a result, a dampingcontroller only has to be applied for one variant, and every othervariant has the applied method. This permits the application expenditureto be reduced significantly. The oscillation behavior of the motorvehicle can be adjusted in an optimum way without excessive damping orinsufficient damping occurring.

The invention also relates to a control device for executing the methodaccording to aspects of the invention. The control device carries outthe method on the control side and for this purpose has means,specifically both hardware means and software means.

The hardware means include data interfaces in order to exchange datawith the 23 assemblies which are involved in the execution of the methodaccording to aspects of the invention. The hardware means also include adata memory for storing data and a processor for processing data. Thesoftware means include program modules for executing the method.

What is claimed is:
 1. A method for adjusting a damping force of shockabsorbers connected between a vehicle body and a wheel at vehiclecorners of a motor vehicle, the method comprising the steps of:determining at least one damping force (F_(LIFT), F_(PITCH), F_(ROLL)),which is referred to a center of gravity of a vehicle body, and which isdivided between shock absorbers of the respective wheels of the motorvehicle as a function of at least one variable which represents amovement of the vehicle body or a movement of the respective wheel, anddetermining, for at least one of the lifting, pitching and rolling modaldirections, the respective damping force (F_(LIFT), F_(PITCH), F_(ROLL))as a function of a predefined, constantly maintained degree of dampingof the respective modal direction.
 2. The method as claimed in claim 1,wherein, for all the lifting, pitching and rolling modal directions, therespective damping force (F_(LIFT), F_(PITCH), F_(ROLL)) is determinedas a function of a predefined, constantly maintained degree of dampingof the corresponding modal direction.
 3. The method as claimed in claim1, wherein, for the lifting modal direction, the degree of damping forthe lifting modal direction is kept constant, in that a damping constantfor the lifting modal direction is adapted as a function of a changinglifting-spring stiffness or changing mass of the motor vehicle, with theresult that the degree of damping for the lifting modal directionremains constant.
 4. The method as claimed in claim 3, wherein thedamping constant d_(LIFT) for the lifting modal direction is adapted insuch a way that the degree of damping D_(LIFT) remains constantaccording to the equation:${D_{LIFT} = \frac{d_{LIFT}}{2\sqrt{c_{LIFT}m}}},$ where D_(LIFT) isthe degree of damping for the lifting modal direction, d_(LIFT) is thedamping constant for the lifting modal direction, c_(LIFT) is thelifting-spring stiffness, and m is the mass.
 5. The method as claimed inclaim 4, wherein the damping constant for the lifting modal direction iscalculated in a continuously updated fashion.
 6. The method as claimedin claim 1, wherein for the pitching modal direction, the degree ofdamping is kept constant in such a way that a damping constant for themodal direction is adapted as a function of a changing pitching-springstiffness or a changing pitching moment of mass inertia of the motorvehicle, with the result that the degree of damping for the pitchingmodal direction remains constant.
 7. The method as claimed in claim 6,wherein the damping constant d_(PITCH) for the pitching modal directionis adapted in such a way that the degree of damping D_(PITCH) remainsconstant and according to the equation:${D_{PITCH} = \frac{d_{PITCH}}{2\sqrt{c_{PITCH}J_{PITCH}}}},$ whereD_(PITCH) is the degree of damping for the pitching modal direction,d_(PITCH) is the damping constant for the pitching modal direction,c_(PITCH) is the pitching-spring stiffness, and J_(PITCH) is thepitching moment of mass inertia.
 8. The method as claimed in claim 7,wherein the damping constant for the pitching modal direction iscalculated in a continuously updated fashion.
 9. The method as claimedin claim 1, wherein, for the rolling modal direction, the degree ofdamping is kept constant in such a way that a damping constant for therolling modal direction is adapted as a function of a changingrolling-spring stiffness or a changing rolling-moment of mass inertia ofthe motor vehicle, with the result that the degree of damping for therolling modal direction remains constant.
 10. The method as claimed inclaim 9, wherein the damping constant d_(ROLL) for the rolling modaldirection is adapted in such a way that the degree of damping D_(ROLL)remains constant and according to the equation:${D_{ROLL} = \frac{d_{ROLL}}{2\sqrt{c_{ROLL}J_{ROLL}}}},$ whereinD_(ROLL) is the degree of damping for the rolling modal direction,d_(ROLL) is the damping constant for the rolling modal direction,c_(ROLL) is the rolling-spring stiffness, and J_(ROLL) is the rollingmoment of mass inertia.
 11. The method as claimed in claim 10, whereinthe damping constant for the rolling modal direction is calculated in acontinuously updated fashion.
 12. A control device for adjusting adamping force of shock absorbers which are connected between a vehiclebody and a respective wheel at vehicle corners of the motor vehicle,wherein said control device is configured to: determine at least onedamping force (F_(LIFT), F_(PITCH), F_(ROLL)), which is referred to acenter of gravity of the vehicle body, and which is divided between theshock absorbers of the respective wheels of the motor vehicle as afunction of at least one variable which represents a movement of thevehicle body or a movement of the respective wheel, and determine, forat least one of the lifting, pitching and rolling modal directions, therespective damping force (F_(LIFT), F_(PITCH), F_(ROLL)) as a functionof a predefined, constantly maintained degree of damping of therespective modal direction.